1. Field of the Invention
The present invention relates to a thin film transistor for a liquid crystal display and a method for manufacturing the same, and more particularly, to a thin film transistor for a liquid crystal display and a method for manufacturing the same capable of decreasing the number of required photomasks.
2. Description of the Related Art
In an information-oriented society these days, the role of electronic displays is becoming increasingly important. The electronic displays of all kinds are widely used in various industrial fields.
Generally, the electronic display is an apparatus for visually transmitting a variety of information to a person. That is, an electrical information signal output from various electronic devices is converted into a visually recognizable optical information signal presented on electronic displays. Therefore, the electronic display serves as a bridge for connecting the person and the electronic devices.
The electronic display is classified into an emissive display, in which the optical information signal is displayed by a light-emitting method, and a non-emissive display, in which the optical information signal is displayed by an optical modulation method, including light-reflecting, dispersing and interfering phenomena. Examples of the emissive display, referred to as an active display, include a CRT (Cathode Ray Tube), a PDP (Plasma Display Panel), an LED (Light Emitting Diode) and an ELD (Electroluminescent Display). Examples of the non-emissive display referred to as a passive display, include an LCD (Liquid Crystal Display), an ECD (Electrochemical Display) and an EPID (Electrophoretic Image Display).
The CRT used in an image display such as a television receiver or a monitor, has the highest market share to date with respect to display quality and economical efficiency, but the CRT also has many disadvantages such as heavy weight, large volume and high power consumption.
Meanwhile, due to rapid development of a semiconductor technology, various kinds of electronic devices are now driven by lower voltage and lower power, which produces electronic equipment that is considerably slimmer and lighter. Therefore, a flat panel type display having these slimmer and lighter characteristics, as well as lower driving voltage and lower power consumption characteristics, is very desirable according to the new environment.
The LCD among the various developed flat panel type displays is much slimmer and lighter than any other displays; LCDs also have low driving voltage and low power consumption, as well as displaying quality similar to that of the CRT. Therefore, the LCD is widely used in various electronic equipment.
The LCD comprises two substrates respectively having an electrode, and a liquid crystal layer interposed between the two substrates. In the LCD, a voltage is applied to the electrode to re-align liquid crystal molecules and to control an amount of light transmitted through the molecules. These LCDs are classified into a transmission type LCD, for displaying an image using an external light source, and a reflection type LCD, for displaying an image using natural light.
One of the LCDs, which is mainly used nowadays, is provided with the electrode formed at each of the two substrates and having a thin film transistor for switching power supplied to each electrode. Generally, the thin film transistor (referred to as TFT, hereinafter) is formed at one side of the two substrates.
Generally, a substrate on which TFTs are formed is referred to as a “TFT substrate.” And, such a TFT substrate is generally manufactured by a photolithography process using a photomask; currently, for example, seven sheets of photomasks are required at the present.
FIG. 1 is a sectional view of a conventional reflection type TFT LCD.
Referring to FIG. 1, after depositing a single layered metallic film or a double layered metallic film such as chromium (Cr), aluminum (Al), molybdenum (Mo) or an alloy of Mo and tungsten (W) as a gate film on a transparent substrate 10 made of glass, quartz, or sapphire, the gate film is patterned using a photolithography process to form a gate wiring (using a first mask). The gate wiring includes a gate electrode 12, a gate line connected to the gate electrode 12 and a gate pad 13 that receives a signal from the outside and transmits the received signal to the gate line.
A gate insulating film 14 made of silicon nitride is formed to a thickness of about 4,500 Å on the substrate on which the gate wiring is formed. A semiconductor film made of amorphous silicon is deposited on the gate insulating film 14 and then patterned to form an active pattern 16 of a TFT (using a second mask).
A metal film is deposited on the active pattern 16 and the gate insulating film 14 and then is patterned using the photolithography process to form a data wiring (using a third mask). The data wiring includes a source electrode 18, a drain electrode 19 and a data pad (not shown) for transmitting an image signal.
After depositing an inorganic passivation film 20 made of silicon nitride on the data wiring and the gate insulating film 14 to a thickness of about 4,000 Å, the inorganic passivation film 20 and the gate insulating film 14 on the source electrode, gate wiring and data pad are dry-etched by the photolithography process (using a fourth mask).
A photosensitive organic passivation film 22 is deposited to a thickness range of about 2–4 μm on the inorganic passivation film 20 and is then exposed using a photomask (using a fifth mask). At this time, the organic passivation film 22 placed on the source electrode 18, gate wiring and data pad is fully exposed.
In addition, to make the reflection plate of the pixel region in a light scattering structure, the organic passivation film 22 is again exposed (using a sixth mask). At this time, the organic passivation film 22 of the display region is incompletely exposed in an irregular pattern having a line width corresponding to the resolution of an exposing machine.
Subsequently, the exposed organic passivation film 22 is developed to form an irregular surface having a plurality of concave portions and convex portions in the organic passivation film 22 and a first via hole for exposing the source electrode 18 and a second via hole for exposing the gate pad 13. In addition, although not shown in the drawings, there is formed a third via hole for exposing the data pad together.
On the organic passivation film 22, in which the aforementioned via holes are formed, a reflection metal film such as aluminum (Al) is deposited and then patterned to form a pixel electrode 26, which is connected to the source electrode 18 through the first via hole, and a gate pad electrode 27, which is connected to the gate pad 13 through the second via hole (using a seventh mask). In addition, there is formed a data pad electrode (not shown), which is connected to the data pad through the third via hole, together. The pixel electrode 26 is formed within the pixel region enclosed by the gate wiring and the data wiring and is provided as a reflection plate.
To manufacture a TFT according to the aforementioned conventional method, the photolithography process is used in the seven layers of the gate wiring, active pattern, data wiring, inorganic passivation film, organic passivation film and pixel electrode and thus at least seven sheets of photomask are needed.
As the number of photomasks used in the photolithography process increases, the more the manufacturing cost and the probability of process error increase. Since this causes an increase in the manufacturing costs, there has been proposed a method for forming the inorganic passivation film as a single layer by deleting the inorganic passivation film in order to simplify the process.
FIG. 2A to 4B are sectional views for describing a method for forming a via hole in a TFT in accordance with another conventional method in which the inorganic passivation film is deleted. Here, FIGS. 2A, 3A and 4A show a part of the display region and FIGS. 2B, 3B and 4B show a part of the pad region.
Referring to FIGS. 2A and 2B, after depositing an organic passivation film 45, made of a photosensitive material, on a substrate 40 on which a gate wiring 42, made of a first metal film, a gate insulating film 43, made of an inorganic insulating film and a data wiring, made of a second metal film, are formed in the order named, via hole portions 45a and 45b of the organic passivation film 45 are exposed using a photomask 30.
Referring to FIGS. 3A and 3B, the exposed via hole portions 45a and 45b of the organic passivation film 45 are developed and removed to form an organic passivation film pattern 45c. Afterwards, the gate insulating film 42 placed below the removed via hole portions 45a and 45b is dry-etched using the organic passivation film pattern 45c as an etch mask to form the first via hole 46 for exposing the data wiring 44 and the second via hole 47 for exposing the gate wiring 42. At this time, the inorganic insulating film is side-etched and thereby an undercut “A” is generated beneath the organic passivation film pattern 45c. 
Likewise, in the case that the data wiring 44 is formed of a material having a high consumptive rate, such as molybdenum (Mo) or MoW, the data wiring 44 is side-etched at an edge of the first via hole 46 and thus the undercut “A” is generated beneath the organic passivation film pattern 45c. At the same time, the data wiring 44 is consumed by a predetermined thickness at the bottom “B” of the first via hole 46.
Referring to FIGS. 4A and 4B, after depositing a reflection metal film such as aluminum (Al) on the organic passivation film pattern 45c in which the first and second via holes 46 and 47 are formed, the deposited reflection metal film is patterned by a photolithography process to form the pixel electrode 48, which is connected to the data wiring 44 through the first via hole 46, and the pad electrode 49, which is connected to the gate wiring 42 through the second via hole 47.
At this time, due to the undercut that is formed beneath the organic passivation film pattern 45c, a failure in the coverage of the reflection metal film is generated and thereby an opening failure of the reflection metal film occurs at the bottom of the first and second via holes 46 and 47.
Accordingly, it is inevitably required to resolve this undercut problem. If the undercut problem is not resolved, it is difficult to use the passivation film as a single layer of organic insulating film and thus it becomes impossible to decrease the number of photomask layers that are needed.